1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device having a LiTaO3 substrate. In particular, the present invention relates to a surface acoustic wave device using surface acoustic waves, the dominant component of which is longitudinal waves (pressure waves or P-waves), that have a phase velocity less than that of “fast shear waves” and “slow shear waves” of bulk acoustic waves (BAWs) propagating in the LiTaO3 substrate.
2. Description of the Related Art
High-performance, light-weight, and small SAW devices are widely used in band-pass filters of portable communication devices. The operating frequency F of SAW devices is determined by F=V/L, wherein V is the phase velocity of the surface acoustic waves and L is the finger period of an interdigital transducer (IDT).
Accordingly, SAW devices having the same operating frequency have an increased finger period L if the phase velocity V is increased. Since the width of the electrode fingers of the IDT is increased as the finger period L increases, it is possible to manufacture SAW devices at high yield and low costs. However, when the period L is excessively large, the chip size increases and the number of SAW devices manufactured from one wafer decreases, thereby increasing the manufacturing costs.
On the other hand, when the phase velocity V decreases, the period L decreases, and the size of SAW devices is reduced. However, when the period L is excessively small, the width of the electrode fingers of the IDT becomes excessively small, thereby requiring costly micro-fabrication to make the IDT.
The phase velocity of the SAWs of the SAW devices must be optimized for their particular use. Moreover, recent communication devices having higher frequencies require higher operating voltages. Thus, SAW devices now require surface acoustic waves having higher phase velocities.
The bandwidth of resonator SAW filters, which are a category of SAW devices, is dependent upon the electromechanical coupling coefficient kS2. The electromechanical coupling coefficient kS2 increases as the bandwidth of the filter increases. Similarly, the electromechanical coupling coefficient kS2 decreases as the bandwidth of the filter decreases. Therefore, the electromechanical coupling coefficient kS2 must be optimized according to the usage. For example, radiofrequency (RF) SAW filters of cellular phones require an electromechanical coupling coefficient kS2 of approximately 4% to 10%.
The loss resulting from propagation of surface acoustic waves, i.e., the propagation loss, increases insertion losses of SAW filters and the resonant resistance of the SAW resonators, and decreases the impedance in the impedance/antiresonant frequency ratio. Thus, the propagation loss is preferably as small as possible.
Known surface acoustic waves for use in SAW devices include Rayleigh waves and leaky waves, most of which have a higher phase velocity than Rayleigh waves. High-frequency SAW devices frequently use leaky waves which have a shear (transverse) wave component that is parallel to the SAW propagation direction, i.e., the u2 component, as the dominant component and which propagate in 36°–42° Y-X lithium tantalate (LiTaO3) substrates or 41° or 64° Y-X lithium niobate (LiNbO3) substrates. Such leaky waves propagate at a velocity of approximately 4,000 to 4,500 m/sec in SAW devices having aluminum IDTs on these substrates.
Recently, second leaky surface acoustic waves, which have a higher phase velocity, i.e., approximately 5,000 to 7,000 m/sec, and which have a longitudinal wave component, i.e., the u1 component, as the dominant component have drawn much attention.
A non-patent document entitled “Longitudinal Leaky Surface Acoustic Waves on Li2B4O7”, Shingaku Shunki Zendai A443 (1994), discloses leaky surface acoustic waves having a longitudinal-wave component as the dominant component and propagating in a lithium tetraborate substrate at a phase velocity of 6,656 m/sec. Another non-patent document entitled “Characteristics Of Leaky Surface Acoustic Wave Propagation On LiNbO3 and LiTaO3 Substrates”, S. Tonami, Y. Shimizu, J. Appl. Phys. Vol. 34 (1995), pp. 2664–2667, discloses second leaky surface acoustic waves having a longitudinal wave component as the dominant component and propagating in LiNbO3 and LiNbO3 substrates. According to the latter non-patent document, the electromechanical coupling coefficient kS2 reaches the maximum, i.e., 2.41%, in a LiTaO3 substrate with an Euler angle of (90°, 90°, 31°), and the propagation losses on a free surface which is electrically open (also referred to as the “open surface”) and on an electrically short-circuited metallized surface (also referred to as the “short surface”) are, respectively, approximately 0.06 dB/λ and approximately 0.5 dB/λ, wherein λ represents the wavelength of the second leaky surface acoustic waves. That is, the propagation loss is greater on the metallized surface than on the free surface. The temperature coefficients of delay (TCD) at the free surface and the metallized surface at the same Euler angle were, respectively, approximately 35 ppm and approximately 48 ppm.
Japanese Unexamined Patent Application Publication No. 8-288788 discloses a SAW device that uses longitudinal-wave-type SAWs in which the longitudinal component is dominant over the shear component. In particular, it discloses a SAW device including a LiTaO3 or LiNbO3 substrate cut at a particular Euler angle and a thin film disposed on the substrate, in which the product of the wave number K of the longitudinal SAWs and the thickness H of the thin film is controlled within a predetermined numerical range. For example, with a substrate composed of LiTaO3 and a conductive thin film made of gold, longitudinal SAWs having a phase velocity less than those of “fast shear waves” and “slow shear waves” are effectively used to reduce the propagation loss to nearly zero by controlling KH to KH=0.6 (H/λ=9.6%) or more.
The “slow shear waves” and the “fast shear waves” are both bulk waves that propagate in a piezoelectric substrate. Three types of bulk waves that propagate in piezoelectric substrates are known as “slow shear waves”, “fast shear waves”, and “longitudinal waves”. The conventional technologies described above teach that the longitudinal SAWs having a phase velocity less than those of the “fast shear waves” and “slow shear waves” have no propagation loss.
For the purpose of this specification, the “slow shear waves”, the “fast shear waves”, and the “longitudinal waves” are defined as follows. Shear waves (transverse waves) are categorized into vertical shear (SV) waves having a u3 displacement component and horizontal shear (SH) waves having a u2 displacement component. SV and SH waves having low acoustic velocities are defined as “slow shear waves” whereas SV and SH waves having high acoustic velocity are defined as “fast shear waves”. Pressure waves or P-waves having a u1 displacement component are the “longitudinal waves”.
As described above, RF SAW filters of cellular phones require an electromechanical coupling coefficient kS2 of approximately 4% to 10%. However, the electromechanical coupling coefficient kS2 of a SAW device including a LiTaO3 substrate and an aluminum conductive film is only approximately 2.13%.
Moreover, conductive films composed of aluminum require increased thickness. Thus, the yield is reduced.
On the other hand, the phase velocity of the longitudinal quasi SAWs disclosed in the above-described patent publication are decreased to approximately 3,300 m/sec, which is nearly the same as the velocity of the Rayleigh waves, to eliminate the propagation loss at the metallized surface. This is done by increasing the thickness of the gold conductive film so as to make the longitudinal quasi SAWs slower than the fast and slow shear waves. However, in this manner, the longitudinal waves no longer have a high phase velocity and cannot meet the requirements of higher frequency.
As is clear from the above description, conventional SAW devices using SAWs having longitudinal waves as the dominant component suffer from low electromechanical coupling coefficient kS2 and high propagation loss. Moreover, they are incompatible with higher phase velocities.